Manufacture of silicate-coated papers



e-iii Presets SR Kill' LKNUL ilitiiiih Patented July 28, 1953 UNITEDQFFICE Philadelphia Quartz Company, Philadelphia, Pa., a oorporationofPennsylvania.

No Drawing. Application January.: 23,1947;

xSerial No. 7233885,V

2 Qlaims. 1 This invention relates to improvements in A,t1-iemanufacture of silicate-coated` papers; and il?. comprises a process ofcoating paperwherein a paper board or sheet, containing ligijieousl.0.1iv

other components normally subject to by the action oi alkali, is coatedwith a` silicate solution having a gravity within lthe range; of about10 to 55 B. and containing a small amount of a bleaching agent andanalkali metal. silicate having a Weight `ratio; 0f .alkali metal oxideto SiQz Within.; the range oliv from about; 1:1,6 to 1:4, the higheraravities beine usually employed with the higher` ratios; of lalkali.metal oxide to S102, the bleaching agent beingi ad vantageously presentin amount; suicients-to. Dro.- duce a bleaching action substantiallyequivalent to that producedA by from. about. 0.25 kto 5.40 per cent ofhydrogen peroxide, whereby the discoloration of the coated papernormally. caused by' the alkaline silicate solution will bedecreased andthe paper will have a brightness which is higher than that of thesheetor board coated with alkaline silicate alone and whichfmay be.

higher than that of theuncoated sheet. Thelmvention also includes. thecoated paper sheet or board produced by the describedprocess, said sheetor board being. supercially bleachedso;

that the discoloration normally caused byl the; alkaline sodium silicateis decreased or e1im.- inatedthe dried reaction produotsioi thebleach.-- ing action being present in said sheet; or board; all as. morefully hereinafter set forth andas claimed.

There has long been an important demand in.

also useful for Wrapping rubber tires and` for preventing various typesof rubber goods vfrom adhering to their containers. Such papers must beresistant to mineral oils, edible oi1s, aromatic. oils, butter,lubricating greases, etc. of these uses the color or brightness` ofthese papers is of secondary consequence provided that they aresu-iciently oilandv greaseproof. But in the case of papers usedl for thewrap-- ping of foodstuffs or in the making of cartons for this purpose,color is,A of coursev, of great importance.

It has long been known that it is possible to increase the greaseresistance of paper Aandpaper boards by oatinethem with silicatesolutions.

lungen-uws F01" Some also been coated- :or impregnated with silicatevsolutions primarily in -order toincrease their stifinessor `ori-.1shi-nestrength; also for the -purpose .of decreasing the penetration andspread of secondary coatings, which .are usually vmuch lllole expensiveA.their silicate coatings. These methods however.; have been ,seriouslylimited in theirapplicationsl.due to `the fact zthatalmost all paperstocks.I ...especially thesuheaper grades, contain corriponents.\W,l:ricl;1.l are subject to alkali stai.nini-,v that. ,is V oo rriponentswhich `produce ,an obgectonal yellow or brown; discoloration whencontacted with the-.alkali present, nsilioate solutions. lt Possiblev toAavoid this staining.- by the `use oi .-puriiiedpulpsor puits`consisting .sub Stantiallyofi oellulosic. components. But thehighoostoi these pulpsmakes thisrmethod `prohibitive for finest industrialapplications. Prior tothe present inyentzionnofloW-.oost :methodwasknown of producingv papers vand paper boards resistant tool-leandgreases or of. increased stiffness and/ or otdecreased .permeabilityfrom :low-arado. paper stocks without impairment of their brightness.The present-.invention is, ofcourse, not limited to-low gradeepanersibut is. vappl-ioablein the .case o iallraners,containing materialswhich ,are disoolored-by alkaline :salu,tions-` The parer nulos whichare most widely .used

for thomas as.. of -oarersand paper :boards ofv the; .type describedabove arey (l.) the sofcalled mechanicalor groundawood nulps preparedbyenndinsfthe:woodstoohmithout chemical treatv mont,Y (.2.) :the.semieohemioal nulnsproduoed; by

, a combination @chemical-,and mechanical; steps..

(.3.) :the chemical nulnss-sulnts., sulfate and sodaefin which theoriginal; -wood chipsfare thor.- ouehly-cookedrto-a puin. chemicalsolutions and Whlhr. for. this purpose.,are not today vusuauyfully-bleached even though yused tor 'high-grade stocks; and 24);ite-worked. mixtures; fof the aforesaid nulos.,Y The first .ci the.above yclasses of f nulos: contains substantially pall; of; the:original non-cellulose constituents of, the wood vfrom .inoludingglieninand, tannin alli-al,.whilearll` classes.Y contain atleast somecomponents subiect to alkali staining, which Components. can; `be:removed only Vby. thorougheoing. puriffyingsoperations.. Itis. :evidenttherefore-paliers yricade;trom these: nulos.: usuallyvcaminante-:coatedgbyanyitreatmentin Wlu'chtheiY are :contacted witalkaline solutions without orodueinsdeteriorat n .colon Ill tldl1--` frtion some papers which are to be Ycoated have of the prior art if smallamounts of bleaching 4` agents are added to the silicate solutionssuicient in amount to counteract part or all'of the staining tendency ofthe alkali present in the silicate solutions and, if desired, sufficientto improve the color or brightness of the paper. order to produce thisresult certain critical conditions must be employed and precautionsobserved. Thus, I have found that the silicate:

solutions must have viscosities such that penerolls which remove theexcess. Or the solution can be applied to the paper by means ofconventional transfer rolls followed by removal of the excess. If anyexcess is left, the coating will be too heavy and it is likely to becomebrittle and crack. Thecoating operation is preferably conducted atordinary temperatures but higher temperatures can be used if necessary.The coated sheet can be given a secondary coating, if desired, eitherwith more silicate or with some other material, or it may be rewounddirectly into a roll.

In lieu of the above methods, it is possible, of course, to employ anyof the usual methods of applying coatings to sheet materials, such asdipping, spraying, brushing operations or by the use of knife or rollspreaders or the like. The

Y, gravities and viscosities of the solutions to be tration of the paperis only partial, that is, viscosities advantageously within the range offrom about 1.0 to 50 centipoises at 20 C. The silicates employed musthave ratios of alkali'metal oxide to SiOz within the range of 1:1.6 to1:4. The silicate solutions should contain from about 8 to 40 per centsilicate solids and` should have gravities within the range'of about 10to 33.5 B. for an alkali-silicate ratio of 1:4 and about 17 to 55 B. fora ratio of 1:'1.6.` lWithin the` range of silicate ratioswhicharesatisfactory, the more alkaline silicates have higher total solids atthe Vsame viscosities. The solutions should be applied with a spread offrom about 2 to 30 pounds per 1000 squarefeet of surface. These spreadswill usually leave a dry Weight of silicate of soda of about 0.3 topounds per 1000 square feet of surface, depending on the structure andhydrationof the sheet. `They should contain sufficient oxidizing agentto' produce av bleaching action substantially equivalent to Athatproduced by from about 0.25 tov 5.0* per cent by Weight of hydrogenperoxide. The solutions may be stabilized if desired with a small amountof a soluble magnesium salt and/or a soluble pyrophosphate.

The above described coating solutions can be applied to the paperA atany point after the web is formed and the bulk of'the Water removedtherefrom. If desired, theA coating solutions can be applied to the webby meansof one or more of the final water boxes of the paper machine.This can be accomplished, for example, by allowing the solution to flowinto a water box on `the calender stack. The web is run through Ythissolution, the excess solution being squeezed. off. The paper then passes.through the rst set of calenderv rolls to another water box where itreceives another treatment with the coating solution and the paper thenpasses through the rest of the calender stack. The bleachingv actiontakes place while the paper machine isV running at full speed, such as500 feet per minute, for example. The silicate solids collect in theouter layers of the paper and 'are dried in place by passage through therest of the calendar.

. It is also possible to apply the silicate solution as the web passesthrough the so-called size press. At such a stage higherviscosity'coating solutions are required. The paper at this point maycontain as much as 90V per 'centof water.

Somewhat better results Vare obtained if my coating solutions areapplied in aseparat'e operation to the paper. A somewhat moreconcentrated solution of silicate can be used if this is done. 'I'hesheet of paper may be' passed through a. tank of this solution and'thenthrough squeeze used must be governed to some extent by the method ofapplication employed. For example when 'applied in a separate operationthe viscositi'es oimy coating solutions should fall within theupper'part of the range of 1.0 to 50 centipoises Whereas, if thesolutions are applied by means of the'water boxes of a paper machine,viscosities within the lower part of this range usually produce bestresults. And allowance must be made for the moisture content of thepaper, the degree of sizing, the type of sizing, etc. If the viscosityis too high, the penetration will be poor, if it is too low, the paperbecomes so wet that it isdifficult to handle.` The viscosity which issatisfactory on a dense hard finished paper is considerably lower thanthat which will give proper'penetration into a porous soft board toproduce stifening-thereof. No hard and fast rules can be given whichwillit all cases but it isbelieved that the description given herein willbeadequate `to enable those skilled in this art to obtain satisfactoryresults with my process.

The papers and paper boards produced `in the' manner described arehighly resistantto oil andgrease, as Well as to waxes, lacquers, etc.,which may be used as outer coating'materials in combination with mysilicate' coating procedure. They are also resistant to water and areconsiderably stiffer than the uncoated sheets or boards. They materiallyreduce the loss of aromatic oils from products wrapped therein andabsorption and/or penetration of oils and greases is substantiallyprevented. If a flexible.

paperis desired, this can be produced by the use of silicate solutionshaving ratios of alkali metal oxide to S102 approaching 1:2 and/or bythe incorporation of glycerine or other plasticizing agents in thecoating solutions.

Ifga paper is desired which is substantially water-proof as well as oiland grease resistant, thiscanbe accomplished by coating my papers withan outer, coating of paran or other waterprooiing material after theyhave been treated by the process described above. My silicate coatingvprocess is particularly advantageous when used as a pretreatment inconnection with other coating procedures for `the reason-that thesilicate jcoating definitely limits the penetration andsoftening of thepaper by other coatings, such asparaffin, otherv waxes and lacquers, forexample. r:Chis `means that a substantial savingis eiected in the amountof auxiliary coating material required to' produce a continuous lm.

My present process should not be confused @www process, wherein paperv;pulp...is bleached inf-bulk,

a small amount o'f silicate, 'usually"upftof8.;per

cent, based on the weight `of the pulp 'may be added to inhibitcorrosion, to increase the `pene.- tration and to stabilize :thebleaching agent. 'But this silicate is washed out in .subsequentvoperations and no silicate coating remains to stiften the paper crrender it oil Vand grease proof. The residual bleach .liquors aredarkbrown or even black. It is also true that the-quantity of .bleachingagent required for a :given weight of v, paper fiber is much greater inthe usual .bleaching process, presumably because in my process the webis `bleached only on its surface and the bleaching solution is moreconcentrated owingto less water being present. Thus, in -thefbulkbleaching process the 4pulp is bleached while in suspension 'in water,only VJfrom about 3 `to 10 per cent of pulp being present, whereas incon-` trast, my silicate coatings are applied to the web when the latterusually7 containsno more than 20 percent of water. This difference inprocedure produces certain surprising resultsj For example, if it shouldbe attempted 'to bleach pulp in the conventional manner employ-l ing theproportions of oxidizing agent to pulp which are used in thepresentprocess, little or no improvement in color would be obtained and, in thecase of sodium hypoohlorite, for example, the brightness of the paper`would actually be reduced. In addition it is necessary in the bulkbleaching process promptly to 'remove all residues of the bleachingreaction in order to obviate a substantial reversion in color whereas,in contrast, in my new process the residues are left in the paper and,surprisingly, vthe reversionin color is small even over a period ofseveral months, much smaller than that which would be Vproduced by theomission Vo-f the washing step in the bulkbleaching process.Another-sur-` prising fact is that the bleaching action is accomplishedin the present `process Within a periodof one or two seconds at amaximum whereas in the bulk process bleaching may require up to severalhours.

It should be noted that in my process the bleaching solution usuallypenetrates the sheet only partially, starting from` the outside. The

bleaching agent which contacts the outer -fibersl has a maximum strengthWhile any 'that penetrates beneath the surface fibers is partly 'spent.V

The innerl fibers of the sheet therefore 'cannot be ybleached to `thesame degree as the surface bers even though they may be partly coatedwith the silicate. In other words there is a `definite gradation in thebleaching of the fibers from the sur-face to the'interior. Since thesurface ibers alone are visible this is no disadvantage, and, of course,the saving in bleaching agent which this makes possible is highlyimportant.

. A-n important advantage of my newprocess is that use can be made ofequipment 'already present in the papermill orl equipment which can beacquired at reasonable cost. On the other hand, a plant for thebulk-bleaching of ground wood pulp requires an investment of .atleastabout $100,000 for a capacity of 50 tons per day. In addition, .aspointed out above, iin my Vvnew process the consumption of bleachingagent isv substantially less.

Any of the oxidizing agents can be used in my process which are usefulin lthe bulk bleaching of paper pulp containing ligneous and `other,`v

materials subject to .alkali staining. These toxid-izing .agents are.those which are :capable of dissolving :and acting :in @alkaline rsolution Ito remetal oxides-of h'igh -valence, such .as the 'halo--gena-tes, mit-rates, arsenates, Aantimonates and the like; thepolythionates and the products of solution of Agaseous oxidizing agents,such-as C12Q, C102 and NO3 :in alkaline solution. Ofcourse, the sameproducts formed by other means, such asx'ithe formation of NaOCl by theprecipitation 0fv-Ca(fOH).2.irom bleaching powder by the addition ofcaustic, `are included. Of these compounds, those equivalent to thenitrates, chlorates, nitrous lacid .and oxygen itself in `alkalineYsolution are ineffective unless used in high concentration and-aretherefore less desirable. Those compounds which release nascent oxygentend 'to be more effective than those which pro.- duce nascent chlorine.

My invention can be described more specifically` by reference to thefollowing examples which represent practical embodiments of .myY

coating process vapplied to the .production of-my oilvand greaseproofpaper.

Eazample n1 vtaining approximately y6.3% :Na20 and 24;6%

SiOztweight ratio ctNazO S102 .of 1:32).. This mixture had Ta gravity-ofabout l27"- B-. and aviscos-ity of about 9.0 centipoises 'at 20 C. andan H2O2 content of about 1%. This solution was surprisinglyqstable andit .was iound that 0.6% H2O2 was still present after standing 24 hours.A sheet of Apaper made from groundwood pulp was `dipped into thissolution. Alike sheet was dipped into a similar solution to which noperoxide had been added.` Another sheet was dipped in water.Thesheetswere passed through squeeze rol-1s tor-emove excess liquid andthe sheetsrwere then dried. The sheet which was dipped in water ,onlyhad a .cream-gray color. That dipped into the diluted "S brandsilicatewas -a vdark brown. That dipped into the S brand silicate solutioncontaining 1% H2O2 was light yellow. Samples of these sheets weretested'on an Ives Tintwphotometer.

- VThe scale of this standard instrument reads from O-100 and is soadjusted that each of `the filters (red, green, blue) reads 100 whentested with a white fresh surface of MgO. The readings indicate thevparts of red, green `and blue light reilected by the sample. A weightedaverage indicating brightness is obtained by 'summating '70% of green,20% of blue and 10% of red. The followingvalues were obtained:

' t BrightlRed Green -l3lue Ile-SS 80% "S Brand with 1% H2O2;

It is evident that treatment-Withwater little o1" no effect butthatthepresence .of 1% H202 increasedthe kwhiteness somewhatover that ofthe original sheet while the :sheet treated :with

siii'cst'e aioe' was very dark. In as months the brightness of thesilicate coated sheet had decreased only 3 points. i

Example 2 In this test N brand sodium silicate was used. This containsapproximately 8.9% NazO and 28.7% SiOz (weight ratio 1:3.22). The fourfollowing coating solutions were prepared:

Parts ofso vH202 soin.. 3.a 3.3 16. 5 1e. 5V Parts of War/1er 3?.'171?.'17 8.2 P tH O a rox.

Pxf 'l 00.0 so. o 60.0 80.0 Degrees Baume 25 34 25 34 Viscosity(approx.) cp. (at C.) 4. 0 20.0 4.0 20.0 Stability, percent H201 after24hours 0. 4 0.3 3.2 2.2

Test samples of the same groundwood pulp paper as used in Example l weredipped into these coating solutions, the excess being removed by squeezerolls as before. The coated samples were then dried and tested on theIves Tintphotometer with the following results:

1 2 3 4 Original Visual color Yellow.. Yellow.. Cream. Cream; Creamgray.Tint-photometer: Red 66. Green 55. Blue 49. Brightness. 55.

Example 3 Another series of wood pulp paper sheets were coated withsolutions of sodium silicates containing Various types of bleachingagents and the resulting products tested as before with the IvesTint-photometer, with the following results:

Coating Solution Red Green Blue Bgslt' 1. Distilled water 55 50 41 40 2.20 p. water and 80 p. "N

brand (blank) 30 20 14 20 3. 1.94 p. NaIOs, 20 p. Water, 80 p.

N 35 24 17 24 4. 3.45 p. NaBOa, 2H1O, 20 p. H2O

80 p. N brand 49 34 20 33 5. 1.33 p. NaClOz, 20 p. water 80 N brand 4528 18 28 6. (2,12 bubbled into 20 p. water and 80 p. N" brand 36 24 1924 7. C102 bubbled into 20 p. water 80 p. N brand 36 23 14 23 8. 40 p.H2O, 60 p. N brand (blank). x w. 43 28 '19 28 9. 4l 4.7 a o p.

151) brag?) 50 35 20 34 NaCl 40 2 lo qli" brand 2f 71 52 34 50 It isevident that all of the above coating solutions, which contain an addedoxidizing bleaching agent, produced some improvement in the color of thetreated paper sheets in comparison with test sheets treated withcorresponding silicate solutions containing no oxidizing agent. Randomconcentrations were used in theY above tests and my experience has shownthat increased brightness can be obtained either by increasing thedilution of the silicate or by increasing the amount'of oxidizing agentpresent. These tests indicate the definite superiority of hydrogenperoxide as an oxidizing agent.

Example 4 In thisy example my process is used in connection with a papermachine producing a 10 point paper containing ground-wood pulp. Acoating solution is prepared by diluting 4 parts of N brand silicatesolution with 1 part of water and adding'2 per cent of hydrogenperoxide. This solution is placed in the pan of a separate coatingmachine which receives the paper delivered from the paper machine, coatsit with the said solution, presses it to remove the excess, leaving aspread of about 10 pounds solution to 1000 square feet of surface, driesit and then 'winds it on a roll. The so-coated paper has a brightnessslightly greater than that of the uncoated paper. The color issubstantially stable over a period of several months. The paper issatisfactorily oiland greaseproof. V'Ihe consumption of silicate andhydrogen peroxide in this coating process amounts to about 26.6 poundsof the N brand silicate and 0.7 pound of H2O2 for each pounds of drypulp. It is therefore evident that the process is relativelyinexpensive.

Example 5 In this example a coating solution is prepared by mixing 1part of N brand silicate solution with 1 part of Water and adding 5 percent of hydrogen peroxide. This solution, having a gravity of about 21.8B., is fed into two water boxes on the calender stack of a paper machineproducing a 16 point paper from a mechanical pulp. The paper web is runthrough these water boxes and the excess silicate is squeezed off. Afterpassing through the first water box and the first set of calender rollsit passes through the second water box where it receives another coatingof the same silicate solution. The paper then passes through the rest ofthe calender stack. The total application is about 2 pounds of solutionper thousand square feet of surface. The bleaching of the surface fibersis very rapid and takes place while the paper machine is operating atfull speed, namely 350 feet per minute. The silicate solids, amountingto about 0.4 pound per 1000 square feet of surface, are concentrated inthe outer fibers of the paper and dried in place by the action of thecalender rolls. The resulting sheet is oil and grease resistant and hasa color which is almost white.

Example 6 An illustration of the manner in which silicate coatings canbe used to impart stiffness to papers and paper boards and to reduce thespread of a subsequently applied water-proof coatinfg is tobe found inthe art of makingrpaper milk bottles. The paper used in making thesebottles should have a high compression Vstrength and itis highlydesirable that this' paper be nonporous in order to reduce the yquantityof the water-proof coating required. Both of these desirable propertiescan be obtained by coating the paper with a silicate solution. The coloror brightness of the paper used is, of course, important and the presentinvention can be ef- Ifectively utilized in such manner that thebrightness 'of the; paper' is either improved or at least maintained atits original value.

In one specic embodiment of this invention as 'applied to the stock usedin making milk bottles, a' good grade of manila board, consisting ofsulfite and some bleached sulfate pulp is dipped into a solution of Sbrand silicate of soda solution (ratio l Na2O:3.9 SiOz) diluted to 29 B.and having a viscosity of about 15 centipoises to WhichV is added 1 percent of H2O2. After dipping, the board is passed through a pair of rollsto remove excess silicate and then dried at-room temperature. Part ofthe board treated in this fashion is coated with a polyvinyl typeplastic to vmake it water proof while another part is coated withparafnn in the usual manner.

Milk bottles made from a silicate-impregnated, paranin-coated stock havea dry crushing strength of at least lO per cent greater than those madefrom the same stock Without the silicate impregnation. But asilicate-impregnate'd', polyvinyl-coated stock produces milk bottleshaving 'a wet crushing strength averaging about 100 per cent greaterthan astock coated with paraiiin alone. The brightness of the stockimpregnated with silicate is equal to or slightly higher than that ofthe original uncoated stock, due to the` presence ci H2O2 in the coatingsolution. The quantity ofY polyvinyl resin required to water-proofsilicate-impregnated stock in the manner described is so loW-due to thedecrease in penetr'a'tioliA caused by theI silicate-that the methoddescribed is practical. The reception, by the trade of milk bottlesWater-proofed in this manner, `is excellent'.

E-:rample 7 An illustration of asilicate coating procedure used toproduce a paper board of increased stiffness and Water resistance is inthe making of memorandum pads fromivhich pencil notations can be removedmerely by wiping with a damp cloth.- In this Aapplication it isessential, of course, that the pad have a good White color.

In a specific embodiment of making such pads, a stiff 26 pointcombination paper board or Bristol board with a substantially Whitesulflte facing sheet and a manila back is impregnated or coated with a Bbrand silicate of soda solution diluted to 13.5 B. and about 2centipoise viscosity to Which is added sodium peroxide in an amountcorresponding to 0.5 per cent H2O2. The excess solution is removed bypassing the board through squeeze rolls. The original whiteness of thefacing sheet is retained and the stiliness of the stock is increased by10 per cent by he impregnation.

Somewhat thinner paper boards are used in the manufacture o' filingseparators or filing cabinet partitions. Ir" the stock used in makingthese articles is impregnated with silicate in the same manner, thestiilness is increased by about 20 per cent without detrimental effecton the color.

The coating procedures described above can be varied in such manner thatthe spread of solution obtained is within the range of about 2 to 30pounds of solution per 1G66 square feet of surface. And ythe process canbe applied to combined paper board and to paper in sheet form, as wellas to papers made from semi-chemical pulps or raw or partially bleachedchemical pulps. Likewise paper sheets such as the iibrous sheets madefrom such impure cellulose raw materials as b-agasse, pineapple, ramie,esparto, straw bers, etc., may be coated within the purview of thisinvention.

Cotton even though it may be unbleached, is usufalfly' so nearly purecellulose. that it is not seriously discolored bythe action of alkalies.During the process the materials which ordinarily would be discolered`by the alkaline solutions are -destroyedfor' converted into materialswhich do not discolor.- TheA use of very small amounts of AhydrogenperoxideA is ysuiiicient to maintain the lcolor of: -the'uncoated sheetwhile the brightness` of V.the sheet can be improved by theV use of'coating -solutions containing lsomewhat larger amounts of oxidizingagent ranging up to about -5 per cent by Weight.

Ifl it is-necessary to operate my coating process vaft temper-aturesvVabove room temperature,V Afor exam-plof when the Water boxes of a papermachine are employed toV apply the coating solution, it is desirable toadd a stabilizing agent thereto. Any Vofthe stabilizing agents,l whichareuseiul in producing stabilization of silicate bleaching baths.- canbe employed in my coating solutions. rIihe most Vsatisfactory ot theseagents are magnesium silicate; magnesium sulfate andsodiunpyrophosphate, alone yor in admixture LAny Water solubllemagnesiumand pyrophosphate `.salts are'suitable.-

In'V the'` case `of one coating solution containing '3:3 parts of a-30percent hydrogen peroxide solution, 17.7 'parts of water and '80 partsof N `sodium silicate solution,` I- found that the addition of 0.8 partof NarPzOmlGHgO yand-.0.2 part of MgS0r-7I-I2O stabilized` the solutionto such an' extentthat its oxidizing power at the end of four days wassubstantially as great as that of `a `similar solution free from thestabilizing agents :after standing '24 hours;y rllhisshows What asurprising stabilizing Yeiiect is produced by these agents.

Itis also true that my coating solutions are substantially morerfstablewhen made from purined sodium'silicate solutions'. Thus, use may be madeof these-calledl E brand silicate, a clarilied grade having the-samelratio' of NazO-'z SiOQ as N brand'. -In one 't'es't -I found, forexample, that a coating solution made from this silicate, after dilutionto 20 B. and viscosity of 2.5 centipoises, and originally containing 5.2per cent H2O2, retained 4.2 per cent of H2O2 after 24 hours and 1.5 percent after 4 days. In another test Star brand silicate was used. Thiscontains approximately 10.5 per cent of NazO and 26.3 per cent SiOz,with a weight ratio of NazO to SiOz of 112.5. When 5.6 per cent of H2O2was added to this silicate, 3.2 per cent was retained after 24 hourseven though the silicate was used undiluted at 42 B.

While I have described what I consider to be the best embodiments of myprocess and product, it is possible, of course, to vary the procedureswhich have been described to a considerable extent Without departingfrom .the purview of this invention. Thus, as mentioned previously, usemay be made ci any of the usual methods of applying coating solutions toa paper web. Various additions can be made to silicate coatingsolutions, such as naked mica and talc, Which decrease the permeabilityof the coated sheets or boards. It is only necessary in order to obtainthe benefits of my process to apply the coating solution at some pointafter the web is formed and after the bulk of the Water is removedtherefrom. One or both sides may be coated. It is obvious, ofcourse,that my coating solution should not be Washed away once it is applied.By this method it is possible for the first time to silicate- 1 l i coata paper lsheet containing ligneous and other materials, normally subjectto staining by silicate solutions, with the use of a single coatingsolution of an alkali metal silicate and a bleaching agent capable ofreleasing nascent oxygen or chlorine, to produce a sheet which is notonly brighter or less discolored than I"sheets coated with the saidsilicate solution alone but which may actually be improved inbrightness, and Without the necessity yof washing out the products ofthe bleaching reaction. It is believed that this is the first time thisresult has been accomplished.

What I claim is:

1. In the manufacture of greaseproof paper from low-cost paper stockscontaining lignin and tannin components subject to staining by alkali,the process which comprises applying to a Web of ysuch a paper stockwhile it is in the process of being made on a paper making machine, froma box mounted on said paper making machine and at a point where said webcontains no more than about 20% of water by weight, a sodium silicatesolution, having a viscosity within the range of from about 1 to 50centipoises, sufficiently low to cause the solution to penetrate theWeb, to increase its stiffness, to decrease'its porosity and to renderit'oil and grease resistant; said silicate solution having agravitybetween about 10 and 55 B., containing a weight ratio of NazO 'toS102 within the range of from about 1:1.6 to 1:4 and containing fromabout 0.25 to 5% of hydrogen peroxide and as a Istabilizing agent amixture of a water soluble pyrophosphate and a Water soluble magnesiumsalt, whereby the brightness of the coated surface of the web is atleast as high as that of a web made from the same paper stock withoutapplication of said silicate solution; the silicate solution beingapplied so that no excess is left on the surface and with a spreadranging fromabout 2 to 30 pounds per 1000 square feet of web surface;and drying the so-applied coating without removing lthe coating orwashing out the residues of the bleaching reaction.

2.l In the manufacture of greaseproof paper from 10W-cost paper stockscontaining lignin and tannin components subject to staining by alkali,the process which comprises applying to a Web of such a paper, which webcontains Vno more than about 20% Water by weight, a sodium silicatesolution, having a Viscosity within the range of from about 1 to 50centipoises suiiiciently low to cause the solution to penetrate the web,to increase its stiiness, to decrease its porosity and to render it oiland grease resistant; said silicate solution having a gravity betweenabout 10'and 55 B., containing a Weight ratio of NazO to lSiOz Withinthe range of from about 121.6 to 1:4 and containing a colorlessoxidizing-bleaching agent of the type which bleaches by liberating anascent gas, selected from the class consisting of chlorine and oxygen,in amount equivalent, with respect to bleachability, to about 0.25 to5.0% of` hydrogen peroxide, whereby the brightness of the coated surfaceof the web is at least as high as that of a web made from the same paperstock without application of said silicate solution; the silicatesolution being applied so that no excess i'sleft on the surface and witha spread ranging from about 2 to 30 pounds per 1000 square feet of websurface; and drying the so-applied coating Without removing the coatingor washing out the residues of the bleaching reaction.

WILLIAM STERICKER.

References Cited in the le of this patent V c UNITED STATES PATENTSNumber

1. IN THE MANUFACTURE OF GREASEPROOF PAPER FROM LOW-COST PAPER STOCKSCONTAINING LIGNIN AND TANNIN COMPONENTS SUBJECT TO STAINING BY ALKALI,THE PROCESS WHICH COMPRISES APPLYING TO A WEB OF SUCH A PAPER STOCKWHILE IT IS IN THE PROCESS OF BEING MADE ON A PAPER MAKING MACHINE, FROMA BOX MOUNTED ON SAID PAPER MAKING MACHINE AND AT A POINT WHERE SAID WEBCONTAINS NO MORE THAN ABOUT 20% OF WATER BY WEIGHT, A SODIUM SILICATESOLUTION, HAVING VISCOSITY WITHIN THE RANGE OF FROM ABOUT 1 TO 50CENTIPOISES, SUFFICIENTLY LOW TO CAUSE THE SOLUTION TO PENETRATE THEWEB, TO INCREASE ITS STIFFNESS, TO DECREASE ITS POROSITY AND TO RENDERIT OIL AND GREASE RESISTANT; SAID SILICATE SOLUTION HAVING A GRAVITYBETWEEN ABOUT 10* AND 55* BE., CONTAINING A WEIGHT RATIO OF NA2O TO SIO2WITHIN THE RANGE OF FROM ABOUT 1:1.6 TO 1:4 AND CONTAINING FROM ABOUT0.25 TO 50% OF HYDROGEN PEROXIDE AND AS A STABILIZING AGENT A MIXTURE OFA WATER SOLUBLE PYROPHOSPHATE AND A WATER SOLUBLE MAGNESIUM SALT,WHEREBY THE BRIGHTNESS OF THE COATED SURFACE OF THE WEB IS AT LEAST ASHIGH AS THAT OF A WEB MADE FROM THE SAME PAPER STOCK WITHOUT APPLICATIONOF SAID SILICATE SOLUTION; THE SILICATE SOLUTION BEING APPLIED SO THATNO EXCESS IS LEFT ON THE SURFACE AND WITH A SPREAD RANGING FROM ABOUT 2TO 30 POUNDS PER 1000 SQUARE FEET OF WEB SURFACE; AND DRYING THESO-APPLIED COATING WITHOUT REMOVING THE COATING OR WASHING OUT THERESIDUES OF THE BLEACHING REACTION.